1 Answer
1

Germanic languages often form compound words like designtime, so the meaning is readily understandable. As a matter of convention, however, English writers typically write them as separate words except for very common compounds:

As a member of the Germanic family of languages, English is special in that compound words are usually written in their separate parts. Although English does not form compound nouns to the extent of Dutch or German, noun phrases like "Girl Scout troop", "city council member", and "cellar door" are arguably compound nouns and used as such in speech. Writing them as separate words is merely an orthographic convention, possibly a result of influence from French.

This Ngram shows that the compound word designtime has become more common in the last 20 years, although writers still prefer the phrase design time by about 30 to 1. Therefore, while it's reasonable to use the single word in parallel with runtime, it's not preferred usage.